The present invention relates to jitter measurements for digital data communication testing.
Characterizing the transient behavior of high-speed digital circuits, i.e. the transition from a logical zero to a logical one and vice versa, has become increasingly important for designing as well as manufacturing such digital circuits. Timing problems can cause single transmission errors, or temporary or even permanent outage of an entire communication system, and have to be avoided. The standard overall figure of merit for a communications system is the Bit Error Rate (BER), however a high value of BER does not necessarily indicate timing problems, as there are many other potential sources of error in a system (for example level/threshold mismatch).
One of the key specifications of high-speed circuits with respect to timing is Jitter. ITU-T G.701 defines jitter as short-term non-cumulative variations of the significant instants of a digital signal from their ideal positions in time. The significant instant can be any convenient, easily identifiable point on the signal such as the rising or falling edge of a pulse or the sampling instant. By plotting the relative displacement in the instants between an ideal pulse train and a real pulse train that has some timing jitter, the so-called jitter function is obtained. In addition to the jitter time function, the jitter spectrum can be displayed in the frequency domain.
Jitter can also be displayed using so-called Jitter-Histograms showing the likelihood for a transition. Jitter Histograms can be measured using a BER Tester, such as the Agilent® 81250 ParBERT® provided by the applicant Agilent Technologies. The histogram values are obtained from a BER vs. Sample Delay measurement (generally referred to as the so-called bathtub curve) by taking the absolute value of the derivative.
More information about jitter is disclosed in the Application Note 1267, “Frequency agile jitter measurement system”, 5963-5353E, Apr. 1995, Hewlett-Packard.